Shale gas and shale oil that have recently attracted attention as new energies are present in a shale layer located deep under the ground at a depth of thousands of meters, and a drilling rig having a drill part 100 as illustrated in FIG. 10 is used in drilling work to extract the shale gas and the shale oil. This drill part 100 includes a drill bit 110 at a tip thereof (close to the shale layer), and is configured to rotate the drill bit 110 via a hollow drill string (not shown) coupled to an upper end of the drill bit 110 using a ground motor (not shown).
A high axial load in an axial direction of a drill occurs at the drill bit 110 when an underground hard bedrock is drilled, and a high axial load caused by a dead load of the drill string is further applied to the drill bit 110. For this reason, a bearing 120 that supports the drill bit 110 does not only require a high load capacity, but also should function to receive an axial load applied in a vertical direction. Since there is no space for structurally holding a lubricant in the bearing on the periphery of the bearing 120, since there is also no function to supply oil or grease in the middle of drilling, and since a surrounding environment unusually becomes a high temperature because it is used under the ground at a depth of thousands of meters, there is a fear that lubrication constantly becomes poor. Since muddy water easily invades into the bearing, there is a possibility of the bearing being damaged in its early stage by water and foreign materials, and the bearing 120 is under a very harsh use environment.
A multi-row ball bearing illustrated in, for instance, Patent Document 1 is used as the bearing 120 for supporting this drill bit 110. In the multi-row ball bearing, numerous inner ring raceway surfaces are formed by making an inner ring wider, and numerous outer rings are arranged to cause outer ring raceway surfaces thereof to face the inner ring raceway surfaces provided on the inner ring. Two sets of multi-row ball bearings are used, one of which receives the axial load from the tip of the drill bit 110, and the other of which supports the dead load of the drill string. To this end, the multi-row ball bearings are disposed to cause directions of contact angles to face each other.
However, since the inner ring is formed in an integrated type in which the multi-row inner ring raceway surface is formed, axial load is uniformly received in each row. To this end, the accuracy of a component should be secured to some extent, and grooves should be accurately machined. In the drill part 100, since the bearing 120 is frequently disassembled, inspected and replaced for maintenance, it is difficult to inspect the bearing 120 like the integrated inner ring. If the bearing 120 is damaged and replaced, the entire bearing 120 should be replaced, and maintenance costs are also increased. Further, specifications considering a high axial load, a high-temperature environment, poor lubrication are not also mentioned.